The proposed project aims to analyze various aspects of the morphology and physiology of mammalian extraocular muscles. For this purpose, the "in vitro" preparation of the inferior rectus muscle of the rat which was developed previously is being used. The morphological study is based on the examination of transverse serial sections of the muscle and it is in a preliminary stage. We aim: a) to recognize the various cellular populations present in the muscle and their distribution in the global and orbital layers; b) to verify if systematic morphological variability is present or not along the fiber length; and c) to identify fibers which after being studied electrophysiologically have been marked with dye spots. Labelling of the fibers will be carried out by iontophoretic injections of Fast green. Fibers will be identified according to established criteria (mitochondrial and sarcoplasmic reticulum developments, fibrillar size, etc.) using light and electronmicroscopy or with specific stainings such as cholinesterase reaction and bungarotoxin labelled with fluorescent dyes. Previous examination of the electrical properties and synaptic activities has shown that most of the fibers are typical singly innervated or twitch fibers while a minority is multiply innervated and equivalent to tonic fibers of amphibians. We shall attempt to correlate the physiological properties of these cell types with their morphological structure. The study proposes also to examine the metabolic requirements of the tension evoked in tonic fibers by cholinomimetics. Preliminary results indicate the tonic tension is rapidly reduced by lowering the 02 tension in the incubation medium and slowly depressed by glucose lack. Experiments designed to analyze the effects of metabolic inhibitors and the quantitative relationship between 02 tension and contractility are planned.